1. Field of Invention
The present invention relates to a steering assist system and a steering assist method that applies appropriate steering torque to assist a vehicle to run along a roadway.
2. Description of Related Art
Conventional steering assist systems acquire the shape of the roadway before the vehicle, determine the required steering torque that will allow the vehicle to travel along the roadway, and apply the required steering torque to the vehicle to relieve a driver of the burden of driving. In order to reduce the deviation of the vehicle from the center line of the roadway due to the influence of the incline of the road, some conventional steering assist systems calculate a target lateral acceleration based on an integral of the lane offset, which is the amount of lateral deviation between the vehicle center line and the center line of the roadway, with respect to time to generate a steering torque required to achieve the target acceleration. (See Japanese Patent Application Publication No. 2005-343260 (JP-A-2005-343260), for example.)
In conventional steering assist systems, when the vehicle deviates from the center line of the roadway, a steering torque responsive to the integral is generated so that the vehicle is controlled to move along the center line of the roadway. In such conventional steering assist systems, however, a steering torque may be applied even if no steering assist is necessary, and thus the driver may feel uncomfortable while performing a steering operation. For example, if the driver is driving intentionally off the center line of the roadway, a steering torque is calculated based on the integral even if the lane offset may be small. As a result, the cumulative steering torque increases over time, which may degrade the steering feel. In this way, the conventional steering assist systems have the problem as described above when there is such a time-dependent steering control amount, if not an integral, that increases the applied steering torque the longer the period of time that the actual moving state of the vehicle deviates from the target moving state.